This invention relates to microwave filters and in particular to coupling arrangements for aperture coupled cavity filters.
In highly selective bandpass filters which use adjacent cavity resonators coupled by apertures in common walls, the magnitudes of such couplings are very critical parameters. An example of such a bandpass filter is disclosed in our co-pending International Application No. PCT/AU99/01071.
In order to achieve these necessary critical parameters it is known to provide a high degree of manufacturing precision. However, this solution is unattractive for large filters.
It is therefore an object of the present invention to provide an adjustable coupling aperture arrangement for adjusting the coupling between cavity resonators over a wide range of coupling values, the coupling being adjusted externally using a tool that does not disturb the filter""s characteristics.
According to the invention, in a microwave filter comprising an electrically conductive housing within which is disposed at least two cavity resonators coupled by aperture means in a substantially planar wall common to both said resonators, there is provided an adjustable coupling aperture arrangement including aperture means comprising at least one slit of predetermined dimensions, the at least one slit communicating with a respective access hole in said housing via an associated passageway that lies within the boundary of said wall""s major surfaces, wherein said at least one slit is provided with a movable metal slug that is slideably retained by opposite longitudinal edges of the slit, whereby said slug can be engaged and slideably manipulated by a tool means, introduced into said access hole and guided to said slug via said passageway, into a position in which electrical contact between said slug and said edges of the slit produces a desired change in effective electrical length of the slit.